Microsolvation and Protonation Effects on Geometric and Electronic Structures of Tryptophan and Tryptophan-Containing Dipeptides

Photodissociation spectroscopy of solvated clusters of protonated tryptophan (TrpH+) and dipeptides containing tryptophan (Val-TrpH+, Ala-TrpH+, and Gly-TrpH+) has been carried out at low temperature to investigate the protonation and solvation effects on the electronic spectrum. For the protonated dipeptides, the S1−S0 transition exhibits a substantial red shift due to the stronger interaction between the NH3+ group and the indole π ring. The S1−S0 spectra of TrpH+(CH3OH)n clusters exhibit a drastic change with the number of methanol molecules. This behavior is interpreted in terms of the decrease in the interaction between the ππ* and the repulsive πσ* states. Ala-TrpH+ and Gly-TrpH+ exhibit an extensive spectral change with addition of two methanol molecules. This change is ascribed to a conformational change, which is induced by the insertion of solvent molecule in between the NH3+ group and the indole π ring.